The steady march of Moore's law has brought an ever-increasing level of processing power to not only desktop and laptop computers, but also mobile devices. Like desktop and laptop computers, many of the mobile devices are capable of processing information at rates equaling or exceeding broadband. Having become accustomed to broadband performance in connection with their (e.g., desktop and/or laptop) computers, users of the mobile devices (“mobiles”) have come to expect from cellular networks to provide at least the performance to match the capabilities of the mobiles, and for the most part, network capacity has grown commensurately. The resultant explosive growth in network capacity has been consistent with Cooper's Law. That is, network capacity (e.g., total data delivery per month) has doubled each month since the days of Marconi, and such growth is predicted to continue for at least the foreseeable future.
A breakdown of Cooper's law reveals that a vast majority of the network capacity results from increases in density of large-scale cellular (“macro”) networks, specifically, as a result of adding more macro cells to the macro networks (and thereby maintaining homogeneity of the macro network). Experts and commentators, however, are currently questioning whether further increasing network capacity of the macro networks in this way is practicable, practical and/or judicious.
Instead of adding more macro cells, operators of a macro network may address increases in data demand (i.e., increase network capacity) by supplementing existing grids of macro cells with small-scale cells, such as microcells, picocells and femtocells, particularly in hotspot regions. Supplementing the macro network with the small-scale cells (hereinafter “femtocells”, for convenience) results in a mixture of macro cells and femtocells; such mixture is commonly referred to as a heterogeneous network (“HetNet”). The HetNet can, in principle, provide cost effective data delivery capable of meeting the performance demanded by users.
Unfortunately, to realize such performance (by maximizing throughput among the macro cells and femtocells) interference in the HetNet among the macro cells and femtocells is likely to occur. This interference (“inter-cell interference”), due to in part, the femtocells overlaying the macro cells, can be substantially more severe than among macro cells of a homogeneous macro network.
In the HetNet, it is potentially beneficial to push users into the femtocells because, in general, fewer users utilize the femtocells, and in turn, compete for resources. This potentially imposes a large Signal to Interference-plus-Noise Ratio (“SINR”) penalty on some such users. For example, any of the femtocells may limit access to users who are members of a Closed Subscriber Group (“CSG”). Each of such femtocells (“closed access femtocells”) has a potential to create significant ICI for a user associated with one of the macro cells (“macro cell user”), when, for example, the macro cell user wanders into a coverage area of one of the closed-access femtocell, but is not a member of the CSG. The ICI arises because the macro cell user cannot connect to the closed access femtocell, which could otherwise provide it a large SINR signal. Instead the macro cell user is forced to use signals emanating from a transmitter of the macro cell (e.g., via a macro network layer), which could have a very low SINR. A similar phenomenon happens for any of the femtocells providing open access, where the macro cell user is associated to a Femto cell due to range expansion, for instance, and hence observes significant interference from the transmitter of the macro cell (e.g., via a macro-network layer).
Various techniques for handing the inter-cell interference (“ICI”) in various types of multi-cell wireless communication networks. For example, when transmitters can share data via backhaul, joint transmission/joint processing (“JT/JP”) techniques in both uplink and downlink has been proposed, for example, in long-term evolution (“LTE”). With the JT/JP in the downlink, data is shared, processed and jointly transmitted via coordinated base stations so that interfering links are used as desired links. With JT/JP in the uplink, received data is shared and jointly processed. It has been shown that JT/JP provides significant gains in multi-cell downlink systems, especially to users operating at edges of cells (“cell-edge users”). However, the overhead and cost required to share data between inter-cell base stations may not be affordable in a lot of practical systems.
Standards, such as LTE, focus on other techniques for handing ICI, which, unlike JT/JP, do not require sharing of data between inter-cell base stations (“inter-cell data sharing”). Among such techniques, in the medium and weak interference regime, which is typically observed in homogeneous networks, is coordinated beam forming (“CBF”). CBF treats the ICI as a noise. It has been shown that CBF provides gains to average cell performance and significant gains to cell-edge users in homogeneous networks, which typically see medium ICI.
However, when a strong ICI is present, CBF is strictly sub-optimal. It has been shown in the single-input single-output (“SISO”) scenario and in the multiple-input multiple-output (“MIMO”) scenario that a strong interference may not be harmful. In the SISO scenario and in the MIMO scenario, superposition coding has been suggested for dealing with the strong ICI in two cell, single user (“SU”) systems, and shown significant gains.